1. Field of the Invention
This invention relates to nuclear reactors, and more particularly to reactor cores controlled by movable control rod elements and utilizing mixed oxide fuel.
2. Description of the Prior Art
The use of fissionable plutonium fuel in nuclear reactors, commonly referred to as mixed oxide fuel, is highly advantageous. Recycle of plutonium fuel produced during the fissioning of other nuclear fuels provides a useful energy resource. In this context, plutonium fuel or mixed oxide fuel refers to a combination of fissionable plutonium with other fissionable elements, such as a combination of UO.sub.2 and PuO.sub.2. For example, a typical combination includes uranium fuel having a U-235 concentration of approximately 0.2 to 1.1 percent by weight, and plutonium obtained from reprocessing burned uranium fuel (first recycle plutonium) having approximately a 4.2 weight percent concentration of plutonium. This invention, it will be seen, is applicable to nuclear cores whenever plutonium, of any significant concentration or type, is used, including plutonium from successive recycles.
The inherent characteristics of mixed oxide fuel, however, limit the use of the fuel because it results in increased reactor control requirements. Plutonium has a high absorption cross section, resulting in plutonium competition with the reactor control means, typically control rod elements, for absorption of neutrons. This competition results in a decrease in the worth of the control rod elements. Further, insertion of plutonium in a core produces an increased Doppler and moderator defect, requiring increased control requirements.
As a result of the increased control requirements and decrease in control rod element ability to control reactivity, reactor core designers have provided core arrangements based upon positioning the mixed oxide fuel away from the control rod elements. For example, reactors have been proposed including bundled rod fuel elements which position plutonium bearing fuel rods in the central regions of fuel elements surrounded on their periphery by control rod elements or bars. The peripheral fuel rods bear a more common nuclear fuel, such as uranium. Also proposed have been "checkerboard" arrangements wherein mixed oxide bearing fuel elements are placed adjacent uranium bearing elements. With such arrangements, the control rod elements are inserted only in the uranium bearing assemblies. More recent arrangements have been proposed which orient mixed oxide fuel only at the lower portions of a core having top mounted control rod elements. This results in the control rod elements being in the vicinity of the mixed oxide fuel a decreased amount of time.
These prior art designs, based upon separating the control rod elements or bars from the mixed oxide fuel, thereby disadvantageously limit the amount of plutonium that can effectively be used in a given core. Some have also required additional control rod elements, beyond those required for a primarily uranium bearing core, which is extremely costly.
Further, reactor control is critical from a safety viewpoint. Throughout the nuclear power generating industry extreme care has always been exercised to ensure conservative, redundant, and safe designs. A typical approach to reactor control design has been to assume that under postulated accident conditions, when insertion or scram of the control rod elements is necessary, the control rod element having the highest control element worth in terms of reactivity is stuck out of the core. The worth of the control rod elements in this configuration has been referred to as the "N minus one" (N-1) control element worth. This hypothetical condition, which serves as one basis for nuclear plant design, is used to minimize the possibility of an area of the core maintaining undesirable criticality with a control element stuck out of position.
It is therefore desirable to provide a core which alleviates these limitations heretofore brought about by plutonium or mixed oxide utilization. It is also desirable to increase the amount of plutonium that can be utilized in a given core, while increasing safety margins through decreasing the potential for detrimental effects under an accident condition.